Low-Pressure Mercury Vapor Discharge Lamp and Compact Fluorescent Lamp

ABSTRACT

A low-pressure mercury vapor discharge lamp has a light-transmitting discharge vessel ( 10 ) enclosing, in a gastight manner, a discharge space ( 11 ) provided with a filling of mercury and a rare gas. The discharge vessel ( 10 ) comprises discharge means for maintaining a discharge in the discharge space ( 13 ). At least a part of an inner wall ( 12 ) of the discharge vessel ( 10 ) is provided with a protective layer ( 16 ). The discharge vessel ( 10 ) is provided with a luminescent layer ( 17 ) comprising a luminescent material. The luminescent layer ( 17 ) further comprises inorganic softening particles ( 27 ) with a softening point above 450° C. The size of the softening particles ( 27 ) is in the range from 0.01 to 10 gm. Preferably, the softening particles ( 27 ) are selected from the group formed by strontium borate, barium borate, yttrium borate, yttrium-strontium borate and calcium pyrophosphate.

The invention relates to a low-pressure mercury vapor discharge lampcomprising a light-transmitting discharge vessel provided with aprotective layer and a fluorescent layer.

The invention also relates to a compact fluorescent lamp.

In mercury vapor discharge lamps, mercury constitutes the primarycomponent for the (efficient) generation of ultraviolet (UV) light. Aluminescent layer comprising a luminescent material may be present on aninner wall of the discharge vessel to convert UV to other wavelengths,for example, to UV-B and UV-A for tanning purposes (sun panel lamps) orto visible radiation for general illumination purposes. Such dischargelamps are therefore also referred to as fluorescent lamps.Alternatively, the ultraviolet light generated may be used formanufacturing germicidal lamps (UV-C). The discharge vessel oflow-pressure mercury vapor discharge lamps is usually circular andcomprises both elongate and compact embodiments. Generally, the tubulardischarge vessel of compact fluorescent lamps comprises a collection ofrelatively short straight parts having a relatively small diameter,which straight parts are connected together by means of bridge parts orvia bent parts. Compact fluorescent lamps are usually provided with an(integrated) lamp cap. Normally, the means for maintaining a dischargein the discharge space are electrodes arranged in the discharge space.In an alternative embodiment the low-pressure mercury vapor dischargelamp comprises a so-called electrodeless low-pressure mercury vapordischarge lamp.

It is known that measures are taken in low-pressure mercury vapordischarge lamps to inhibit blackening of parts of the inner wall of thedischarge vessel, which parts are in contact with a discharge which,during operation of the discharge lamp, is present in the dischargespace. Such blackening, which is brought about by interaction betweenmercury and the glass from which the discharge vessel is made, isundesirable and does not only lead to a reduction of the maintenance butalso to an unaesthetic appearance of the lamp, particularly because theblackening occurs irregularly, for example, in the form of dark stainsor dots.

A low-pressure mercury vapor discharge lamp of the type described in theopening paragraph is known from International Application WO-A 01/56350. In the known discharge lamp, the discharge vessel is made from asodium-rich glass, the inner wall of the discharge vessel being providedwith a protective layer comprising a borate or a phosphate of analkaline-earth metal and/or of scandium, yttrium or another rare earthmetal. A luminescent layer is provided on top of the protective layer inthe discharge vessel. The known discharge lamp has an improvedmaintenance.

A drawback of the use of the known low-pressure mercury vapor dischargelamp is that the maintenance still is relatively poor. As a result, inaddition, a relatively large amount of mercury is necessary for theknown lamp in order to realize a sufficiently long service life. In thecase of injudicious processing after the end of the service life, thisis detrimental to the environment.

The invention has for its object to eliminate the above disadvantagewholly or partly. According to the invention, a low-pressure mercuryvapor discharge lamp of the kind mentioned in the opening paragraph forthis purpose comprises:

-   -   a light-transmitting discharge vessel enclosing, in a gastight        manner, a discharge space provided with a filling of mercury and        a rare gas,    -   the discharge vessel comprising discharge means for maintaining        a discharge in the discharge space,    -   at least a part of an inner wall of the discharge vessel being        provided with a protective layer,    -   the discharge vessel being provided with a luminescent layer        comprising a luminescent material,    -   the luminescent layer further comprising inorganic softening        particles with a softening point above 450° C.,    -   the size of the softening particles being in the range from 0.01        to 10 μm.

In a discharge vessel of a low-pressure mercury vapor discharge lampaccording to the invention with a luminescent layer comprising inorganicsoftening particles with an average size smaller than 10 μm, a betteradhesion of the fluorescent layer on the wall of the discharge vessel isobtained. The combination of a protective layer and the fluorescentlayer with the softening particles appears to be very well resistant tothe action of the mercury-rare gas atmosphere which, in operation,prevails in the discharge vessel of the low-pressure mercury vapordischarge lamp. As a result, blackening due to interaction betweenmercury and the glass from which the discharge vessel is manufactured isreduced, resulting in an improved maintenance. During the service lifeof the discharge lamp, a smaller quantity of mercury is withdrawn fromthe discharge, so that, in addition, a reduction of the mercuryconsumption of the discharge lamp is obtained and in the manufacture ofthe low-pressure mercury vapor discharge lamp a smaller mercury dosewill suffice.

Blackening caused by withdrawing mercury from the discharge occurs instraight parts as well as arc-shaped parts of the low-pressure mercuryvapor discharge lamp. In general, blackening is reduced by providing theinner wall of the discharge vessel with a sufficiently adherent andsufficiently thick protective layer. The application of a protectivelayer in combination with a luminescent layer comprising softeningparticles with an (average) size smaller than 10 μm according to theinvention causes the adhesion of the luminescent layer to besubstantially improved in the straight parts as well as the arc-shapedparts of the low-pressure mercury vapor discharge lamp and thisconsequently leads to an improved maintenance. In experiments it isobserved that discharge vessels provided with alumina consume moremercury than discharge vessels with softening particles according to theinvention.

The measure according to the invention is notably suitable for compactfluorescent lamps having arc-shaped lamp parts, wherein the dischargevessel is additionally surrounded by a light-transmitting envelope. Thetemperature of the discharge vessel of such “covered” compactfluorescent lamps is comparatively high because the heat dissipation tothe environment is reduced by the presence of the envelope. Thisunfavorable temperature balance adversely affects the maintenance of theknown discharge lamp due to an increased level of blackening; aninfluence already observed in early lamp life. In experiments it hassurprisingly been found that the maintenance of a compact fluorescentlamp provided with a low-pressure mercury vapor discharge lamp accordingto the invention, the discharge vessel of which is surrounded by anenvelope, exceeds % 98% after 500 burning hours, while the maintenanceof an identical compact fluorescent lamp provided with the knownlow-pressure mercury vapor discharge lamp, the discharge vessel of whichis surrounded by an envelope, is less than % 96% after 500 burninghours.

The addition of a (low melting) so-called frit glass into theluminescent layer for improving the bonding strength of a fluorescentlayer to a discharge vessel of a low-pressure mercury vapor dischargelamp is known in the art. The use of relatively small softeningparticles, with an (average) size smaller than 10 μm, has the advantagethat bonding of the fluorescent layer to the wall of the dischargevessel is improved.

Preferably, the size of the softening particles is in the range from0.01 to 1 μm. By employing sub-micron softening particles, theconcentration of the softening particles in the fluorescent layer can berelatively low.

Preferably, the inorganic softening particles comprise a melting pointabove 450° C. In this way, the adhesion is improved both the arc-sahpedand strait parts of the vessel. In particular, for those lamps in whichbended parts are formed subsequent to coating the fluorescent layer, amelting point of the softening particles around and above 600° C.,improves the adhesion of the fluorescent layer in the arc-shaped partsof the discharge vessel. Other advantages are a lowered Hg consumptionduring life and an improved run-up of the low-pressure mercury vapordischarge lamp.

A preferred embodiment of the low-pressure mercury vapor discharge lampaccording to the invention is characterized in that the softeningparticles comprise a borate and/or a phosphate of an alkaline earthmetal and/or a borate and/or a phosphate of scandium, lanthanum, yttriumor a further rare earth metal. Such softening particles improve thebonding of the fluorescent layer to the glass wall or to pre-coat of thedischarge vessel.

In a preferred embodiment of the low-pressure mercury vapor dischargelamp according to the invention, the softening particles comprisecalcium borate, strontium borate and/or barium borate and/or thesoftening particles comprise calcium phosphate, strontium phosphateand/or barium phosphate. Such softening particles have a relatively highcoefficient of transmission for visible light. Moreover, low-pressuremercury vapor discharge lamps with a luminescent layer comprising suchsoftening particles have a good maintenance.

In a further preferred embodiment of the low-pressure mercury vapordischarge lamp according to the invention, the softening particlescomprise lanthanum borate, cerium borate and/or gadolinium borate,and/or the softening particles comprise lanthanum phosphate, ceriumphosphate and/or gadolinium phosphate. Such softening particles have arelatively high coefficient of transmission for ultraviolet radiationand visible light. It has further been found that softening particlescomprising lanthanum borate or gadolinium borate or comprising ceriumphosphate or gadolinium phosphate have a good adhesion with the innerwall of the discharge vessel.

Very suitable softening particles are strontium borate, barium borate,yttrium borate, yttrium-strontium borate which can be combined withother materials with a higher melting point such as for example Aluminaor calcium pyrophosphate.

A preferred embodiment of the low-pressure mercury vapor discharge lampaccording to the invention is characterized in that the protective layercomprises a borate and/or a phosphate of an alkaline earth metal and/ora borate and/or a phosphate of scandium, yttrium or a further rare earthmetal. Such a protective layer gives a further enhancement of thebonding of the fluorescent layer to the glass wall of the dischargevessel. In a preferred embodiment of the low-pressure mercury vapordischarge lamp according to the invention, the protective layercomprises yttrium oxide or aluminum oxide. Yttrium oxide and aluminumoxide, for example Alon-C, for use as a protective coating inlow-pressure mercury vapor discharge lamps is known in the art.Low-pressure mercury vapor discharge lamps with such a protective layerhave a good maintenance.

In a preferred embodiment of the low-pressure mercury vapor dischargelamp according to the invention, the protective layer comprises calciumborate, strontium borate and/or barium borate and/or the protectivelayer comprises calcium phosphate, strontium phosphate and/or bariumphosphate. Such a protective layer has a relatively high coefficient oftransmission for visible light. Moreover, low-pressure mercury vapordischarge lamps with such a protective layer provide that the phosphorlayer has a good adhesion to the pre-coat an improved maintenance.

In a further preferred embodiment of the low-pressure mercury vapordischarge lamp according to the invention, the protective layercomprises lanthanum borate, cerium borate and/or gadolinium borateand/or the protective layer comprises lanthanum phosphate, ceriumphosphate and/or gadolinium phosphate. Such a protective layer has arelatively high coefficient of transmission for ultraviolet radiationand visible light. It has further been found that a protective layercomprising lanthanum borate or gadolinium borate or comprising ceriumphosphate or gadolinium phosphate have a good adhesion with the innerwall of the discharge vessel and the phosphor layer thus also improvingthe maintenance. The protective layer in the low-pressure mercury-vapordischarge lamp in accordance with the invention further satisfies therequirements with respect to light and radiation transmissivity and canbe easily provided as a very closed and homogeneous protective layer onan inner wall of a discharge vessel of a low-pressure mercury vapordischarge lamp. This is effected, for example, by rinsing the dischargevessel with a solution of a mixture of suitable metal-organic compounds(for example, acetonates or acetates, for example, scandium acetate,yttrium acetate, lanthanum acetate or gadolinium acetate mixed withcalcium acetate, strontium acetate or barium acetate) and boric acid orphosphoric acid diluted in water, while the desired protective layer isobtained after drying and sintering.

In practical embodiments of the low-pressure mercury vapor dischargelamp, said protective layer has a thickness of approximately 5 nm toapproximately 2 μm. At a layer thickness of more than 2 μm, excessiveabsorption of the radiation generated in the discharge space takesplace. At a layer thickness of less than 5 nm, there is interactionbetween the discharge and the wall of the discharge vessel. Such layerthicknesses are customarily obtained by means of optical measurements. Alayer thickness in the range from approximately 50 nm to approximately200 nm is particularly suitable. In said preferred range, the protectivelayer has a relatively high reflectivity in the wavelength range around254 nm.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1A is a cross-sectional view of an embodiment of a compactfluorescent lamp comprising a low-pressure mercury vapor discharge lampaccording to the invention;

FIG. 1B is a cross-sectional view of a detail of the low-pressuremercury vapor discharge lamp as shown in FIG. 1A, and

FIG. 1C is a cross-sectional view of a detail of the wall of thedischarge vessel as shown in FIG. 1B.

The Figures are purely diagrammatic and not drawn to scale. Particularlyfor clarity, some dimensions are exaggerated strongly. Similarcomponents in the Figures are denoted by the same reference numerals asmuch as possible.

FIG. 1A schematically shows a compact fluorescent lamp comprising alow-pressure mercury vapor discharge lamp. The low-pressuremercury-vapor discharge lamp is provided with a radiation-transmittingdischarge vessel 10 enclosing, in a gastight manner, a discharge space11 having a volume of approximately 10 cm³. The discharge vessel 10 is aglass tube which is at least substantially circular in cross-section andthe (effective) internal diameter of which is approximately 10 mm. Thetube is bent in the form of a so-called hook and, in this embodiment, ithas a number of straight parts, two of which, referenced 31, 33, areshown in FIG. 1A. The discharge vessel further comprises a number ofarc-shaped parts, two of which, referenced 32, 34, are shown in FIG. 1A.An inner wall 12 of the discharge vessel 10 is provided with aprotective layer 16 and with a luminescent layer 17 comprising aluminescent material. The discharge vessel 10 is supported by a housing70 which also supports a lamp cap 71 provided with electrical andmechanical contacts 73 a, 73 b, which are known per se. The dischargevessel 10 of the low-pressure mercury-vapor discharge lamp is surroundedby a light-transmitting envelope 60 which is attached to the lamphousing 70. The light-transmitting envelope 60 generally has a mattappearance.

FIG. 1B very diagrammatically shows a cross-sectional view of a detailof the low-pressure mercury-vapor discharge lamp shown in FIG. 1A. Thedischarge space 11 in the discharge vessel 10 does not only comprisemercury but also a rare gas, argon in this example. Means formaintaining a discharge are constituted by an electrode pair 41 a (onlyone electrode is shown in FIG. 1B) which is arranged in the dischargespace 11. The electrode pair 41 a is a winding of tungsten coated withan electron-emissive material, here a mixture of barium oxide, calciumoxide and strontium oxide. Each electrode 41 a is supported by an(indented) end portion of the discharge vessel 10 (not shown in FIGS. 1Aand 1B). Current supply conductors 50 a, 50 a′ issue from the electrodepair 41 a through the end portions of the discharge vessel 10 to theexterior. The current supply conductors 50 a, 50 a′ are connected to an(electronic) power supply which is accommodated in the housing 70 andelectrically connected to the electrical contacts 73 b at the lamp cap71 (see FIG. 1A).

An alternative embodiment of the discharge lamp in accordance with theinvention comprises so-called electrodeless discharge lamps, in whichthe means for maintaining an electric discharge are situated outside adischarge space surrounded by the discharge vessel. Generally said meansare formed by a coil provided with a winding of an electric conductor,with a high-frequency voltage, for example having a frequency ofapproximately 3 MHz, being supplied to said coil, in operation. Ingeneral, said coil surrounds a core of a soft-magnetic material.

FIG. 1C very diagrammatically shows a cross-sectional view of a detailof the wall of the discharge vessel 10 as shown in FIG. 1B. The innerwall 12 of the discharge vessel 10 is provided with a protective layer16 and with a luminescent layer 17 comprising a luminescent material.According to the invention, the luminescent layer 17 further comprisesinorganic softening particles 27 with a softening point above 450° C.The size of the softening particles 27 is in the range from 0.01 to 10μm. Preferably, the size of the softening particles 27 is in thesubmicron range, preferably in the range from 0.01 to 1 μm. Theinventors have found that the average particle size of the softeningparticles 27 and a relatively narrow distribution of the particle sizeof the softening particles 27 play an important role in the softeningproperties of the material. Materials with smaller particles lead to alower melting point of the material. In addition, a smaller size isfavorable for the lower content of material needed to prevent powder offof the fluorescent layer.

Generally speaking, the sintering of the fluorescent layer during themanufacture of the discharge vessel of the low-pressure mercury vapordischarge lamp takes place at temperatures in the range fromapproximately 500° C. to approximately 600° C. In this temperature rangethe binder in the fluorescent layer is removed after coating thephosphor-liquid onto the inner wall of the discharge vessel. Fordischarge lamps in which the bended parts are formed subsequent tocoating the fluorescent layer, the manufacturing of the arc-shaped partsof the discharge vessel takes place at temperatures betweenapproximately 700° C. and approximately 800° C. while softening theglass wall of the discharge vessel. If the softening material is notsoftened at this operation temperature, the luminescent layer is notcapable of maintaining the bonding with the glass vessel and is thuspeeled.

The application of inorganic softening particles in the fluorescentlayer is particularly advantageous for the arc-shaped parts of thedischarge vessel. A softening point of the softening particles above450° C., preferably around and above 600° C., improves the adhesion ofthe fluorescent layer in particular in the arc-shaped parts of thedischarge vessel.

The composition of the softening particles is selected such that thematerial is insoluble in solvents. By properly selecting the material(and the size and the size distribution) of the softening particles, theadhesion of the fluorescent layer to the pre-coat or glass wall of thedischarge vessel can be influenced.

Several material parameters have been tested. First of all, the effectof these parameters on the effectiveness to prevent powder-off isstudied. The various softening materials have been added to the phosphorsuspension in 0.1, 0.5, 1 and 2 wt. % with respect to the solid contentof the luminescent material. The size of the particles of the materialsadded to the luminescent layer is small enough to provide an excellentadhesion of the luminescent layer onto the glass in the straight partsof the discharge vessel as well as in the arc-shaped parts of thedischarge vessel.

A number of low-pressure mercury vapor discharge lamps have beenmanufactured. The protective layer and the luminescent layer are bothapplied by down flushing in the discharge vessel. Subsequently, thelayers are sintered in a furnace around 600° C. in air. In general,employing softening particles with a relatively small size of theparticles (between 0.1 and 1 μm) is much more effective than employingthe same materials with coarser particle size. It was observed that forsoftening particles with a relatively small particle size distributionthe powder off is substantially reduced. On a scale ranging from “0”meaning “no powder off” to “10” meaning “full powder off”, it isobserved that for the arc-shaped parts of low-pressure mercury vapordischarge lamps provided with a luminescent layer with softeningparticles according to the invention the adhesion is “0” whereas as inthe arc-shaped parts of the known low-pressure mercury vapor dischargelamp provided with a luminescent layer without softening particles theadhesion is “8”. In addition, the run-up time of the low-pressuremercury vapor discharge lamps provided with a luminescent layer withsoftening particles according to the invention is in the range from 50to 60 s whereas the run-up time of the known low-pressure mercury vapordischarge lamp provided with a luminescent layer without softeningparticles the adhesion is in the range from 60 to 70 s.

Maintenance results measured over 2000 hours of various low-pressuremercury vapor discharge lamps were obtained. Table I summarizes theresults. From Table I it can be concluded that the maintenance oflow-pressure mercury vapor discharge lamps provided with a luminescentlayer with softening particles according to the invention issubstantially better than that of the known low-pressure mercury vapordischarge lamps provided with a luminescent layer without softeningparticles.

TABLE 1 Adhesion of the phosphor particles in the bends (Delight)Softening particle. 3 × 150 3 × 125 3 × 100 3 × 80 total (mg) SrB₄O₇ 0.5wt % 0 10 5 23 38 SrB₄O₇ 1 wt. % 0 15 5 10 30 SrB₄O₇ 2.0 wt. % 0 7 5 113 0.1% alon-C(ref) 50 24 10 22 106

TABLE 2 Adhesion improvement in straight part of the bulb (delight)Adhesion blow-off Klepper (mbar) (3 × position3) (3 × position5)Sr-borate 0.5% 240 0 1 Sr-borate 1.5% 427 0 1 Ba-borate 0.5% 361 1 1Ba-borate 1.5% 634 0 0 Alon-C 0.1% 130 2 5

TABLE I Maintenance of various low-pressure mercury vapor dischargelamps provided with a luminescent layer with softening particlesaccording to the invention as compared to a luminescent layer withoutsoftening particles. Maintenance (%) luminescent layer with softeningparticles according known luminescent layer hours to the invention nosoftening particles 100 100 100 500 97.7 96.0 Run-up at 100 hours)invention 50-60 sec. And known 60-70 sec.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. A low-pressure mercury vapor discharge lamp comprising: alight-transmitting discharge vessel (10) enclosing, in a gastightmanner, a discharge space (11) provided with a filling of mercury and arare gas, the discharge vessel (10) comprising discharge means formaintaining a discharge in the discharge space (13), at least a part ofan inner wall (12) of the discharge vessel (10) being provided with aprotective layer (16), the discharge vessel (10) being provided with aluminescent layer (17) comprising a luminescent material, theluminescent layer (17) further comprising inorganic softening particles(27) with a softening point above 450° C., the size of the softeningparticles (27) being in the range from 0.01 to 10 μm.
 2. A low-pressuremercury vapor discharge lamp as claimed in claim 1, characterized inthat the softening particles (27) comprise: a borate and/or a phosphateof an alkaline earth metal and/or a borate and/or a phosphate ofscandium, lanthanum, yttrium or a further rare earth metal.
 3. Alow-pressure mercury vapor discharge lamp as claimed in claim 2,characterized in that the alkaline earth metal is calcium, strontiumand/or barium.
 4. A low-pressure mercury vapor discharge lamp as claimedin claim 2, characterized in that the further rare earth metal islanthanum, cerium and/or gadolinium.
 5. A low-pressure mercury vapordischarge lamp as claimed in claim 1, characterized in that thesoftening particles (27) are selected from the group formed by strontiumborate, barium borate, yttrium borate, yttrium-strontium borate andcalcium pyrophosphate.
 6. A low-pressure mercury vapor discharge lamp asclaimed in claim 1, characterized in that the size of the softeningparticles (27) is in the range from 0.01 to 1 μm.
 7. A low-pressuremercury vapor discharge lamp as claimed in claim 1, characterized inthat the inorganic softening particles (27) have a melting point above600° C.
 8. A low-pressure mercury vapor discharge lamp as claimed inclaim 1, characterized in that the protective layer (16) comprisesyttrium oxide or aluminum oxide.
 9. A low-pressure mercury vapordischarge lamp as claimed in claim 1, characterized in that theprotective layer (16) comprises: a borate and/or a phosphate of analkaline earth metal and/or a borate and/or a phosphate of scandium,yttrium or a further rare earth metal.
 10. A low-pressure mercury vapordischarge lamp as claimed in claim 8, characterized in that the alkalineearth metal is calcium, strontium and/or barium.
 11. A low-pressuremercury vapor discharge lamp as claimed in claim 8, characterized inthat the further rare earth metal is lanthanum, cerium and/orgadolinium.
 12. A compact fluorescent lamp comprising a low-pressuremercury-vapor discharge lamp as claimed in claim 1, characterized inthat a lamp housing (70) is attached to the discharge vessel (10) of thelow-pressure mercury-vapor discharge lamp, which lamp housing isprovided with a lamp cap (71).